A diaphragm actuator widely used in automobiles for actuating on-board equipment such as a cruise control system normally comprises a pair of cup-shaped casing halves, and a diaphragm having its peripheral edge clamped between the casing halves, and separating the interior of the casing into two chambers, and a central part of the diaphragm is connected to an object to be actuated. Thus, by introducing a negative pressure into one of the chambers, a corresponding actuating force can be applied to the object to be actuated.
In such a diaphragm actuator, to simplify the manufacturing process and reduce the weight of the moving part of the actuator, an opening formed in a central part of the diaphragm is simply fitted onto a boss projecting from a central part of a disk-shaped retainer plate, and the object to be actuated is connected to the boss via suitable coupling means.
However, according to such a structure, when the pressure difference across the diaphragm abruptly increases, the object to be actuated may not be able to readily follow the deformation of the diaphragm due to inertia or frictional resistance. In such a case, a relatively large tension may be developed in the diaphragm, and the coupling between the central opening of the diaphragm and the boss of the retainer plate may be adversely affected. In an extreme case, the central opening may be stretched to such an extent that air may leak from one chamber to another via the central opening of the diaphragm. Also, if the diaphragm is excessively stretched, it may be folded up, and caught between an edge of the retainer plate and the adjacent wall of the actuator casing.
It is conceivable to securely clamp the diaphragm to a peripheral part of the retainer plate, but it will cause local stress in the part of the diaphragm adjacent to the clamped portion. And, because a substantial part of the diaphragm is constrained, the freedom of deformation of the diaphragm is restricted, and the durability of the diaphragm may be reduced.
Japanese utility model publication (kokai) No. 4-113304 discloses a diaphragm actuator designed to avoid such problems. To prevent excessive stretching of the diaphragm, an annular bulge and an annular bead having an increased thickness are arranged coaxially around the central opening. The annular bead having a substantially rectangular cross section is intended to prevent tensile stress from reaching the central region, and the annular bulge portion relieves the tension and accommodates deformation without causing creases in the diaphragm if it is transmitted beyond the annular bead.
However, this prior proposal only provides a partial solution to the problem, and a better solution to the problems has been desired.